but that isn't the issue.
Nuking a plate or bowl for several minutes will allow the material to be heated up to a point that can burn human skin. Everyone agrees on that.
The ceramic mug, by itself, is not a fire hazard since it won't catch fire in the microwave. If you can set a ceramic mug on fire then that's news to me. Maybe they mean the sparking can catch the mug's coating on fire, but the article doesn't say.
"Chicago to my mind was the only place to be. ... I above all liked the city because it was filled with people all a-bustle, and the clatter of hooves and carriages, and with delivery wagons and drays and peddlers and the boom and clank of freight trains. And when those black clouds came sailing in from the west, pouring thunderstorms upon us so that you couldn't hear the cries or curses of humankind, I liked that best of all. Chicago could stand up to the worst God had to offer. I understood why it was built--a place for trade, of course, with railroads and ships and so on, but mostly to give all of us a magnitude of defiance that is not provided by one house on the plains. And the plains is where those storms come from."
-- E.L. Doctorow
